The invention concerns a tunable cavity resonator as defined in the preamble of claim 1. The invention further concerns a tunable microwave oscillator that uses a cavity resonator of such a kind.
Tunable cavity resonators are used, inter alia, in microwave oscillators that are utilized to generate carrier signals in microwave communication. Such oscillators substantially comprise a microwave amplifier that is operated in feedback, and a high-quality cavity resonator that is located in the feedback path of the oscillator and filters out phase noise generated in the amplifier. A microwave oscillator of this kind furthermore uses a mechanical or electrical phase shifter to adjust the phase condition in the feedback path, and a high-frequency coupler to couple out the useful signal (carrier signal).
Adjustment of the oscillator frequency is accomplished in two stages: For coarse adjustment, first the resonance frequency of the tunable cavity resonator is modified in suitable fashion. This is done by means of the adjustment device with which the position of the tuning plate with respect to the resonator body is displaced. For fine adjustment of the oscillator frequency, the oscillator frequency is then shifted in controlled fashion within the resonance width of the tuned cavity resonator, using the phase shifter to displace the phase in the feedback path of the oscillator.
One difficulty with this type of two-stage toning of an oscillator results from the fact that the maximum frequency excursion achievable by phase adjustment is relatively small, for example only approximately 100 kHz for resonator qualities above 104 (i.e. Q greater than 104). Complete tunability of the microwave oscillator can only be achieved, however, if the minimum frequency change achievable in the context of resonance frequency tuning (i.e. cavity resonator tuning) is less than the aforementioned maximum frequency excursion when varying the phase in the feedback path of the oscillator. To meet this criterion, cavity resonators with an extremely high tuning accuracy are required.
It must be considered in this context that as the quality Q of a cavity resonator increases, the requirements in terms of the adjustment accuracy of the tuning mechanism in order to achieve a defined tuning accuracy also increase.
In practice, therefore, difficulties often occur in terms of the physical design of the tuning mechanism; and it has been found that the desired high adjustment accuracies, in combination with the necessary vibration resistance and good tuning reproducibility, are not always achieved.
The publication entitled xe2x80x9cTemperature compensated high-Q dielectric resonators for long term stable low phase noise oscillators,xe2x80x9d Proceedings of the 1997 Frequency Control Symposium, I. S. Ghosh et al., pp. 1024-1029, describes a tunable cavity resonator as defined in the preamble of claim 1. This cavity resonator, with a quality Q≈105, meets the tuning accuracy requirements necessary for continuous tunability of a microwave oscillator.
DE 1 687 62 discloses an apparatus for adjusting the spacing between a stationary and a movable partition element of a cavity resonator, a lever that is in engagement with the movable partition via a bearing element being arranged rotatably on the stationary partition element. The lever is displaced via a conically tapering segment. The wall of the cavity resonator is thereby moved in order to retune the frequency of the resonator. The linear excursion through which the lever travels at its free end is converted at the wall of the resonator into a reduced linear excursion.
It is the object of the invention to create a cavity resonator that possesses high adjustment accuracy in terms of its resonance frequency. The intention is, in particular, to make available a cavity resonator that exhibits high quality and nevertheless makes possible complete tunability of a microwave oscillator when used therein. A further purpose of the invention is to create a completely tunable microwave oscillator having a high-quality cavity resonator.
The features of claims 1 and 12 are provided in order to achieve the object. The result of the conversion ratio mechanism provided according to the present invention is that upon an actuation of the adjustment device, it is not the linear excursion generated by the adjustment device, but rather a linear excursion reduced with respect thereto, that adjusts the tuning plate. The consequence of this is that the minimum excursion change attainable with the adjustment device is transformed into an even smaller minimum excursion change acting on the tuning plate. As a result, the adjustment accuracy of the tuning plate is increased, compared to the adjustment accuracy of the adjustment device, by an amount equivalent to the predefined ratio of the conversion ratio mechanism. The predefined ratio (i.e. the transmission ratio) is determined by the spring constants of the two spring elements. The use of two spring elements pressing against one another has the advantage that the conversion ratio mechanism operates continuously and in a manner largely free of backlash.
In this instance, a particularly preferred variant embodiment is characterized in that the first spring element is formed from at least one cup spring, and the second spring element is implemented by a plate spring that is immobilized at the periphery and impinged upon centrally by the cup spring. A spring mechanism of this kind can be designed with sufficient stiffness to be insensitive to external shock or vibrations. In addition, suitable cup and plate springs can easily be manufactured with the requisite high spring constants.
The adjustment device preferably comprises an, in particular, manually actuable mechanical actuating element and a first electromechanical actuating element, in particular a first piezoelement, downstream from the mechanical actuating element. The first electromechanical actuating element makes possible electrical activation of the adjustment device, which is advantageous in particular when the adjustment device is operated in a closed-loop mode for adjustment of the resonance frequency xcfx89R. The electromechanical actuating element can also be used, for example, to compensate for temperature-related drift, and can moreover, within a limited excursion range, eliminate the need for an actuation of the mechanical actuating element.
The tuning plate is preferably made of a dielectric material, in particular sapphire. A tuning plate of this kind has very low dielectric losses especially at low temperatures, so that the quality achievable for the cavity resonator (defined as the product of the resonance frequency OR times the quotient of the field energy stored in the resonator and the power dissipation occurring in the resonator) is high (Q≈107).
The positionally adjustable tuning plate according to the present invention can also, in principle, be a wall element (for example the cover wall) of the cavity resonator. A particularly preferred exemplary embodiment of the invention is, however, characterized in that a dielectric element is provided in the resonator body; and that the tuning plate is arranged inside the resonator body at a small distance d from a flat surface of the dielectric element. With a design of this kind, much of the field energy is stored in the dielectric element, and a precise change in the resonance frequency of the cavity resonator can be achieved by means of a change in the position of the tuning plate.
When a dielectric element is used, a further variant implementation that is advantageous in terms of design consists in mounting the dielectric element on a displaceable base whose height can be modified by means of a second electromagnetic actuating element, in particular a second piezoelement. It is thereby possible, without great effort, to define a desired nominal or initial distance between the tuning plate and the flat surface of the dielectric element, which can then be finely adjusted in suitable fashion by the adjustment device according to the present invention with downstream conversion ratio mechanism.
The invention will be explained below by way of example with reference to an exemplary embodiment, with the aid of the drawings, in which: